This invention relates to transmission lines, and more particularly to active terminators for transmission lines.
Cables and longer wiring traces on printed-circuit boards (PCBs) can act as transmission lines. Such lines are treated as transmission lines when the time for a signal to travel from one end to the other end of the line is equal to or more than half of the signal transition (rise or fall) time. High-performance integrated circuits can decrease rise and fall times, while PCB traces remain roughly constant. Thus more signal lines have to be treated as transmission lines in more advanced systems.
Impedance mismatches between the transmission line and its loads cause reflections. Reflection are signals that travel along the transmission line, back and forth between the driver and a receiver, causing unwanted signal ringing, overshoot, undershoot, and noise. The quality of signals is degraded.
Various techniques are used to minimize reflection and ringing. Terminating resistors, resistor-capacitors, and diodes can be added to the transmission line to match line impedances or clamp the undershoots and overshoots of ringing. Active terminators have also been used.
FIG. 1 is a diagram of a prior-art active terminator for a transmission line. P-channel transistor 12 and n-channel transistor 14 have their drains connected to a transmission line. Transistors 12, 14 are biased on so that they can actively terminate the transmission line.
The desired gate biases for transistors 12, 14 is generated by current-mirror transistors 16, 18, which each have their drains coupled to their gates. A continuous D.C. current 11 is generated by a current source, causing current to flow through current-mirror transistors 16, 18 from power to ground. The p-channel gate node from current-mirror transistor 16 is coupled to the gate of p-channel transistor 12. This p-gate node is biased to about one transistor threshold of p-channel transistor 16 below the power-supply voltage. P-channel transistor 12 is biased in the linear region and has a relatively low drain current compared to a similar transistor biased by a larger gate-to-source voltage in the saturated region.
Likewise, n-channel transistor 14 has its gate biased to about one n-channel transistor threshold above ground by n-channel current-mirror transistor 18. Since the n-channel threshold is about 0.6 volt, n-channel transistor 14 clamps the transmission line to about xe2x88x92500 mV. When the transmission-line voltage drops below ground to xe2x88x920.5 volt or beyond, n-channel transistor 14 turns on more strongly in the saturated region, clamping the undershoot. Transistors 12, 14 need to be rather large in area to provide a sufficient clamping current despite being biased in the linear region.
The constant current through transistors 16, 18 is undesirable, since many complementary metal-oxide-semiconductor (CMOS) digital circuits are zero standby power devices. Stability at the gate node is also a problem, since transitions on the transmission line can be coupled to the gate nodes through the drain-to-gate parasitic capacitances of transistors 12, 14. The drain current of transistors 12, 14 can be reduced by the noise coupled into the gate nodes, reducing the effectiveness of the active clamp.
What is desired is a zero-standby-power active termination circuit for a transmission line. A lower-power terminator is desired using CMOS transistors.